According to the general practice of manufacturing the halogen-filled incandescent lamps comprising a translucent enclosure, a helical incandescent body arranged within the enclosure and made of tungsten and means for energizing the incandescent body connectable to an electric supply source are provided. The gas space of the lamp contains a halogen element which is intended to take part in a chemical reaction with the metallic tungsten vaporized from the incandescent body. The vaporized tungsten would be otherwise condensed on different surface parts of the enclosure which are obviously cooler than the incandescent body. The reaction results in a tungsten halide which in given conditions is generally a gaseous compound. The tungsten halide undergoes decomposition on the surface of the incandescent body, thereby transporting tungsten back to the incandescent body and the halogen element is released. The halogen taking part in the cycle described above hinders the process of blackening of the enclosure and ensures an increase of the service life of the tungsten helical body. In order to minimize the effects which adversely affect the cycle, it is very important to manufacture the enclosure in a manner that on its inner surface no area or only a minimized area remains wherein the temperature does not exceed the temperature of condensation of the tungsten halide. The cycle can take place only when conditions are ensured wherein the tungsten halide can easily move within the gas space. While the temperature of the enclosure can be kept without difficulties at such a high value, problems have been encountered in ensuring such high temperatures for the remaining parts of the lamp and especially at the ends of the helical incandescent body. The problem can be solved in principle by two ways, i.e. by passive and active protection.
The passive protection means overdimensioning the parts within the limits allowed by the possibilities, for example either a helical body is arranged as a protecting cover on the so-called interval, i.e. on the coldest parts, or the helical incandescent body is formed in the space surrounded thereby or covered with a thick current conducting wire. Such solutions are described e.g. in United Kingdom Patent 1,254,616 or German Offenlegungsschrift 3,124,218. In the methods of the active protection the temperature is increased by applying appropriate heating means acting on predetermined surfaces.